Manufacturing processes for fabricating semiconductor devices and integrated circuits require the use of chemicals in a gas form with their temperatures precisely controlled. Generally, the gas is generated in a source such as a bubbler in which a carrier gas is bubbled through a liquid chemical to provide a vapor which is mixed with the carrier gas. The temperature of the mixture must be controlled to prevent condensation of the vapor before it reaches the reaction chamber. In some prior art equipment, heating is accomplished by using the delivery lines as a distributed heating element; that is, by passing electrical current through the line to provide resistance heating of the line. There are problems with such heating systems. They require large transformers to supply the necessary heating current. The heated delivery line section must be electrically isolated from the remainder of the equipment. Such couplings introduce a variety of problems, including the introduction of contaminants. The temperature is difficult to control dynamically because of the heat storage capacity of the lines, because the temperature of the gas is not directly measured, and because there is low heat transfer between the lines and the gas.